Filter tow

ABSTRACT

The invention relates to a filter tow made from crimped endless cellulose acetate filaments, whereby the filaments form at least two connected fibre bundles with the same filament titre of at least 600 filaments. The filament titres differ from bundle to bundle by at least 1 dtex Furthermore a filter tow is disclosed with a crimp ratio of more than 40%, said filaments comprising at least 22 crimp turns per filament length.

[0001] The invention relates to a filter tow used in the manufacture of cigarette filters having a comparatively low filtration performance whose draw resistance retention characteristic was selected in such a way that their use in full-flavor cigarettes with a ventilation degree of more than 25% is possible while maintaining an acceptable draw resistance in the cigarette and an acceptable filter plug weight.

[0002] The great majority of cigarette filters currently in use are manufactured from filter tow, which consists of continuous cellulose 2,5-acetate filaments. These filters are referred to as acetate filters. The cigarettes that are manufactured and sold in the European Union can be classified as follows based on their condensate contents in smoke:

[0003] Full-flavor

[0004] Medium

[0005] Light, and

[0006] Ultralight cigarettes

[0007] Because of the maximum-content regulations (90/239/EEC) applying to condensate content, all of these cigarettes are ventilated. The degree of ventilation depends on the condensate target value and on the filtration performance of the filter being used.

[0008] The following table summarizes the typical current values: Class mg Condensate Filament Dernier % Ventilation Full Flavor <12 2.8-3.3 dtex <25 Medium 8-10 ≦3.3 dtex <35 Light 4-8  2.1-2.8 dtex 30-50 Ultralight  <4 1.7-8.8 >50

[0009] Most of products currently sold have a diameter of 7.8±0.2 mm and a cigarette length of 85-120 mm. In addition, there are niche products whose design characteristics differ significantly from the most commonly sold products. In response to this wide variety of product requirements, the filter tow industry supplies a multitude of different tow types that can essentially be described by the following characteristics: filament dernier (dtex) 1.6-8.8; total dernier (dtex) 17,000-60,000; crimp index (%) 25 max. 38; number of crimps per cm: 10 max. 20.

[0010] The following description of the object of the invention exclusively relates to the class of full-flavor products. A typical filter cigarette of the full-flavor class has the following typical characteristics (these values are approximate typical values). They serve simply as a basis for calculations for further embodiments. Lengths: Cigarette:   84 mm Filter:   24 mm Tobacco rod:   63 mm Tipping:   26 mm Diameters: Filter: 7.80 mm Cigarette: 7.85 mm Draw resistance: Filter:   66 mm water column Tobacco rod:   50 mm water column Cigarette:  100 mm water column Filter tow specification: 3.0 Y 35 (filament dernier: 3.33 dtex; total dernier: 38.900, cross-sectional shape of the filaments: Y) Tobacco mixture: American Blend Ventilation: Degree of ventilation: 20% Position of ventilation zone:   11 mm from the mouth end Smoke values: Condensate:   12 mg Nicotine:  0.9 mg Carbon monoxide (CO):   14 mg Retention values: Condensate: 42% Nicotine: 38% Carbon monoxide (CO): None

[0011] As a consequence of the new maximum-content regulation (2000/C 150E/03) of the European Union, full-flavor products in particular will have to undergo drastic changes in their design features, since effective Jan. 1, 2004 not only the condensate value, but also the nicotine value—and in particular the CO value—will be regulated with respect to their maximum limits. The challenge involving the modification of the products to meet the new EU regulations will be that the permitted maximum amounts with an optimal smoke flavor will need to hit the values 10 mg condensate, 1 mg nicotine, and 10 mg CO per cigarette as precisely as possible. In other words, in the future a 1:1 ratio of CO/tar must be targeted for full-flavor products. As the above table shows, the ratio in this example is 1.17.

[0012] Based on the model calculations that solely take a change in filter ventilation into account, it can be assumed that a filter ventilation of approximately 35% will be required to reduce the CO from the current value in the above calculation example of 14 mg to a future value of 10 mg per cigarette. An increase in the degree of ventilation in an otherwise unchanged product will unavoidably necessitate a decrease in the cigarette draw resistance.

[0013] For consumers however, an optimal smoke flavor means that the draw resistance of the cigarette must not fall significantly below the current value of approximately 100 mm water column. Increasing the degree of ventilation, however, will mean that maintaining the other filter parameters such as draw resistance, diameter, position of the ventilation zone, and the filter length will significantly reduce the draw resistance of the cigarette. The resulting cigarette draw resistance can be calculated using the following equation, which is based on a laminar flow model:

P _(V) =P ₀ −V×(P ₀ −P _(F) ×L _(V) /L _(F)), where:

[0014] P_(V): Draw resistance of the ventilated cigarette (mm water column)

[0015] P₀: Draw resistance of unventilated cigarette: =draw resistance [filter+tobacco rod] (mm water column)

[0016] P_(F): Filter draw resistance (mm water column)

[0017] V: Ventilation degree of filter (%)/100

[0018] L_(V): Distance of the ventilation zone from the mouth end (mm)

[0019] L_(F): Filter length

[0020] Thus, the open draw resistance will then only be 88 mm water column (instead of the original 100 mm water column) for the stated sample cigarette at a ventilation degree of 35%. Such a drop in the cigarette draw resistance is clearly perceivable for the smoker and it must be offset by an increase in the filter resistance (in the above example to approximately 80 mm water column) in order to maintain the consumer's smoke feel. This increase in the filter draw resistance results in an unavoidable increase in the CO/tar ratio, since the tar values, but not the CO values, are influenced by the filter draw resistance (an increase in the filter draw resistance results in a decrease in the tar values).

[0021] Therefore, the draw resistance/retention ratios must be redefined for future products in the “full-flavor” category. The adjustment to a 1:1 CO/tar ratio and the filter tow used to manufacture the filter needed to achieve an acceptable product with regard to cigarette draw resistance must be modified to meet these requirements. In this regard, the following specifications may be defined:

[0022] 1. The filter tow must be suitable for producing the draw resistance values that are typical today (however preferably even higher values).

[0023] 2. The filtration performance of the filter made with the filter tow of the invention must, with the given filter draw resistance, be significantly lower than the current typical retention values in order to achieve the 1:1 CO/tar ratio.

[0024] 3. The additional material consumption that results from the increase of the filament dernier must be kept within acceptable limits.

[0025] With a given filter diameter and length, the retention of a filter depends exclusively on the draw resistance and the filament dernier. For example, an increase in the filament dernier and a decrease in the draw resistance results in a drop in filtration performance (and vice-versa). Since the interrelationships are of a very complex nature, filter calculation programs are used to estimate the extent of such changes. One example of such calculation programs is Cabled (Capability Line Expert; copyright Rhodia Acetow GmbH 1998, Freiburg, Germany) from the firm of Rhodia Acetow.

[0026] In the prior art, therefore, the retention of a filter is reduced by increasing the filament dernier. Model calculations suggest that in the future the filament dernier will need to be increased by at least 0.5-1.5 dtex in order to meet the requirements for filtration performance. However, the filter tow specifications commonly encountered in the market (the mean total dernier of all specifications is approximately 35 ktex) are not suitable for also covering the required draw resistance range with the required minimum filament dernier (retention) (Cable© calculation program from Rhodia Acetow GmbH). In the prior art, this problem can only be solved with a monoacetate filter by drastically increasing the filter dernier to well above 45,000 dtex.

[0027] Such solutions, for example a filter tow of the type 4.5 Y 55 (see FIG. 2) in the prior art would then meet the draw resistance and retention specifications, but would result in an increase in the consumption of filter tow of >35% compared with current products.

[0028] The above discussion was limited to a description of the full-flavor group of cigarettes. If, however, a CO/tar ratio of 1:1 is generally sought, the discussion regarding the widening of the filter resistance ranges would apply to all filament derniers. This means that the object of the invention described below will generally comprise the currently standard filament dernier spectrum of 1.6-8.8 dtex.

[0029] Therefore, the object of the invention is to disclose a filter tow with which it is possible to manufacture filters that have a filtration performance achieving a CO/tar ratio of 1:1.

[0030] In order to meet the above requirements, the invention proposes two approaches that are basically independent of each other and whose elements can, if desired, be combined.

[0031] The solution of the invention relates to a filter tow of crimped, continuous cellulose acetate filaments, in which the filaments form at least two coherent fiber bundles that have the same filament dernier and have at least 600 filaments and that differ in their filament dernier from fiber bundle to fiber bundle by at least 1 dtex. In particular, this solution produces outstanding results with regard to filtration performance, since it was unexpectedly found that a filter that is manufactured from such a non-homogeneously distributed mixed tow exhibits a significantly lower tar retention than does a product made of a conventional tow with a uniform filament dernier distribution or a mixed tow having a homogeneous filament distribution (see FIG. 3).

[0032] The second solution is accomplished by a filter tow that has a more intensive and at the same time markedly finer crimp structure than the typical products encountered today. Claim 4 recites a filter tow having a crimp index of more than 40%, in which the filaments have at least 22 curls per cm filament length. In particular, this filter tow fulfills the maximum requirements with regard to filter draw resistance. The values that are achieved are even exceeded if the crimp index is more than 42%, whereby the filaments preferably have more than 25 curls per cm.

[0033] In order at the same time to achieve optimal filtration performance by the filters, a filter tow having a filament dernier of more than 3.33 dtex and a total dernier of less than 44,000 dtex, preferably less than 40,000 dtex is proposed. The limitation on the total dernier results in an increased filter tow consumption that is still acceptable.

[0034] The design and the advantages of the two solutions of the invention may be described as follows: The crimp index Ix is a measure of the intensity of curling. The crimp index of a filter tow is determined by means of the tensile test (load/elongation relationship). It is defined as the ratio of the elongated length L2 under the test load minus the initial length over the initial length under preload L₁:

I _(x)=(L ₂ −L ₁)/L ₁*100(%)

[0035] Here, the test load is 25 N and the preload is 2.5 N. The length between the jaws of the tensile test machine is 250 mm. The curling index is determined in a tensile test at a constant traverse rate of 300 mm/min on a G02 test machine manufactured by Borgwaldt GmbH, Hamburg. 10 individual readings were taken per measurement. Testing is performed in a standard climate: 20° C. and 60% relative humidity. (The test method for determining the crimp ratio is described in the Cable program under the “Help” button).

[0036] The crimp ratio values of current filter tow specifications normally lie between 25 and 35%. The firm of Rhodia Acetow is the sole manufacturer that offers worldwide specifications under the designation SK in a filament dernier range of <2.78 dtex that covers a crimp ratio range of from greater than 35% to a maximum of 38%.

[0037] As already stated, the crimp ratio is a measure of the intensity of crimping and, thus, the position of the performance curve. In this context, a performance curve is understood to mean a performance range of a filter tow, which states the draw resistance range in which a filter tow can be processed with a given diameter and filter rod length under typical processing conditions. For this working range, the performance curve describes the relationship between the draw resistance and weight. The relationship between the position of the performance curve and the crimp ratio is shown using the example of a filter tow of type 2.1 Y 35 HK (filament dernier: 2.33 dtex; total demier: 38,900; cross-sectional shape of the filaments: Y; crimp ratio: 32%) and a 2.1 Y 35 SK (filament dernier: 2.33 dtex; total demier: 38,900, cross-sectional shape of the filament: Y; crimp ratio: 36%) in FIG. 1.

[0038] The above object is accomplished by manufacturing a filter tow that has a crimp ratio of more than 40%, preferably more than 42%. This ratio is absolutely necessary in order to cover the corresponding draw resistance range. The crimp ratio is affected by the frequency and the amplitude of the crimps. The number of crimps provides a measure of the fineness of crimping. Currently, typical values range from 10 to max. 20 crimps per cm filament length. The SK tows referred to above normally have 15-18 crimps per cm.

[0039] It was unexpectedly discovered that such an index can only be achieved with an acceptable filter tow tensile strength if the crimping is hyper-fine. Therefore, the filter tow of the invention has, in addition to an index of more than 40% in the filaments, on average more than 22 crimps per cm, preferably more than 25. Such a filter tow can be processed on conventional filter rod machines into cigarette filters that have a draw resistance level comparable to filters manufactured using the currently typical specifications. FIG. 2 shows the working range of a tow of type 4.5 Y 35 of the invention (filament dernier: 5.0 dtex; total demier: 38,900 dtex) in comparison with a conventional filter tow of type 3.0 Y 35 (filament dernier 3.33 dtex: total dernier: 38,900 dtex) respectively a conventional 4.5 Y 55 and a conventional 4.5 Y 35.

[0040]FIG. 2 clearly shows that, compared with the current situation (3.0 Y 35) significantly more material is consumed with the solution of the invention (Example 1: 4.5 Y 35) as a result of the increase of the filament dernier, but that this additional consumption ends up being significantly less than with a traditional prior-art solution (4.5 Y 55).

[0041] In order to produce hyper-fine crimping without being overly aggressive, certain basic conditions must be met in the manufacturing process. Prior to the crimping process, the filter tow band contains the following components that soften the acetate filaments: water, acetone, and softening components. It is known that these softening components have a lasting effect on the crimping process. For example, U.S. Pat. No. 5,225,277 describes the effect of acetone and tow band temperatures on the stability of crimping. Products having up to 19.7 crimps per cm can be manufactured using the method described therein. This document does not make any statements with regard to the crimp ratio. However, using the data stated therein (draw resistance, tow specification and machine settings) a crimp ratio that is clearly below 35% can be estimated with the help of the Cable software referred to above.

[0042] JP unexamined patent document 56-53223 describes the effects of steam-treating the toll band before it enters the crimping machine. Products having 16 crimps per cm are described. No information on the crimp ratio is provided. It has now been unexpectedly discovered that hyper-fine crimping can be achieved effectively and without all-too drastic damage to the filaments if the total amount of the plasticizing components exceeds a value of 25 weight % at the time the crimping process occurs. The concentrations in which the individual components are present does not matter. The total content of the individual components is determined experimentally using a gravimetric method by taking a sample at 150° C. (20 min.) directly downstream from the crimping machine.

[0043] As already noted, one of the solutions to the object of the invention relates to a filter tow that has a so-called mixed dernier.

[0044] DE 29 16 062 describes such a filter tow and the cigarette filters made from it. In contrast to the filter tow of the invention, in this citation the most uniform possible distribution of the filaments is provided (see column 2, lines 22 ff. in DE 29 16 062). In the embodiment of DE 29 16 062, the effect of this is that the amount of material used in the filters that are to be produced is decreased with the same draw resistance, but as a consequence of the uniform distribution of the filaments the retentions in the filters increase significantly.

[0045] The object of the described invention is also to save material, but at the same time the retention in the filters made from the filter tow should be significantly less than that which is achieved when corresponding filters are made from a filter tow having a uniform filter dernier. This is achieved when measures are taken to ensure that in the product of the invention the filaments of the coherent fiber bundles of the same filament dernier are formed of at least 600 filaments, preferably more than 1000 filaments, that differ in their filament demier from fiber bundle to fiber bundle by at least 1 dtex, preferably by more than 1.5 dtex. The filter tow contains at least two, preferably more than five areas of filament bundles having the same filament dernier. The advantage of this embodiment is that the filament bundles of the same filament dernier agglomerate in the filter and thus form areas of low, respectively high, draw resistance. A filter manufactured from such a mixed-dernier tow accordingly has similar properties with respect to draw resistance and retention as those described, for example, in JP2000000085 for a classical core-jacket filter. There are no restrictions with regard to crimp ratio and number of crimps per cm for this tow filter. However, in a preferred embodiment of the invention, a product having a crimp ratio of more than 33%, preferably more than 35%, is made.

[0046] The advantages of the invention set forth in claims 8 to 10 may be summarized as follows: Based on the combination of materials set forth in the invention, working ranges with respect to draw resistance that are not currently available may be achieved. By applying the teachings of claim 10 with the given draw resistance, the filament demier can be increased so that the products can achieve a significantly lower filtration performance. As a result of the cited restriction with regard to the total demier, the consumption of material can be limited in manufacturing the filters. However, a combination of the embodiments of at least one of claims 1 to 7 with one or more of claims 8 to 10 is especially advantageous. This embodiment permits a further significant reduction of the total dernier to be used and, thus, results in further savings potential with regard to material consumption.

[0047] The invention is described in greater detail below based on the examples.

COMPARISON EXAMPLE (TOW OF TYPE 4.5 Y 55)

[0048] A spinning solution having a concentration of 30 weight percent cellulose acetate in acetone is prepared, filtered, and then extruded through spinnerets. The water content of the spinning solution is 1.5%. The spinnerets are selected with regard to the number of holes, hole size, and number of spinnerets so that ultimately a tow of type 4.5 Y 55 is made. The application of softening agent is set so that an oil coating of 0.5% results. The tow band is crimped with a conventional stuffer box crimping machine so that after drying and packing, a filter tow having a crimp ratio of 30.5% results. The total content of plasticizing components is 22.3%. The tensile strength of the tow is 143 N. The number of crimps is 15.8 per cm filament length.

[0049] The product is processed into filter rods having a length of 126 mm and a diameter of 7.8 mm on a conventional filter rod machine, model KDF2/AF2, make: Hauni, Hamburg. The generation of lint during the processing of the product is insignificant and amounts to 0.5 g/ton for the estimated consumption of one ton of material. The performance curve of the product shown in FIG. 2 covers a draw resistance of 310-460 mm water column.

EXAMPLE 1

[0050] A spinning solution having a concentration of 30 weight parts cellulose acetate is prepared in acetone, filtered, and then extruded through spinnerets. The water content of the spinning solution is 3%. The spinnerets are selected with regard to number of holes, hole size, and number of spinnerets so that ultimately a filter tow of the type 4.5 Y 35 is produced. The application of softening agent is adjusted so that an oil coating of 1.3% results. The resulting tow band is crimped with a conventional stuffer box crimping machine so that, after drying and packaging, a filter tow having a crimp ratio of 43.5% results.

[0051] The total content of plasticizing components is 29%. The tensile strength of the tow is 111 N. The number of crimps is 26 per cm filament length.

[0052] The product is processed into filter rods having a length of 126 mm and a diameter of 7.8 mm on a conventional filter rod machine, model KDF2/AF2, make: Hauni, Hamburg. The generation of lint during the processing of the product is insignificant and amounts to 2.8 g/ton for the estimated consumption of one ton of material. The performance curve of the product, shown in FIG. 2, covers a draw resistance range of 298-505 mm water column.

EXAMPLE 2

[0053] A spinning solution having a concentration of 30 weight parts cellulose acetate is prepared in acetone, filtered, and then extruded through spinnerets. The water content of the spinning solution is 3%. The spinnerets are selected with regard to number of holes, hole size, and number of spinnerets so that ultimately a filter tow of the type 4.5 Y 32 is produced. Unlike in the above examples, the spinning machine is divided into six zones, and alternating zone by zone, a higher filament dernier (1185 filaments 6.9 den=7.65 dtex per zone) and a lower filament dernier (1185 filaments 2.1 den=2.3 dtex per zone) is spun. The application of softening agent is adjusted so that an oil coating of 1.3% results.

[0054] The resulting tow band is crimped using a conventional stuffer box crimping machine so that after drying and packaging, a filter tow having a crimp ratio of 39.8% results. The total content of plasticizing components is 27.5%. The tensile strength of the tow is 129 N. The number of crimps is 23.1 per cm filament length.

[0055] The product is processed into filter rods having a length of 126 mm and a diameter of 7.8 mm on a conventional filter rod machine, model KDF2/AF2, make: Hauni. The generation of lint during the processing of the product is insignificant and amounts to 0.9 g/ton for the estimated consumption of one ton of material. The performance curve of the product covers a draw resistance range of 267-487 mm water column. 

What is claimed is:
 1. A filter tow of crimped, continuous cellulose acetate filaments, wherein the filaments form at least two coherent fiber bundles of the same filament dernier of at least 600 filaments which differ from fiber bundle to fiber bundle in their filament dernier by at least 1 dtex.
 2. The filter tow of claim 1, wherein the filaments form more than five coherent fiber bundles of the same filament dernier.
 3. The filter of claim 1 or 2, wherein the individual fiber bundles contain more than 1000 filaments.
 4. The filter tow of one of claims 1 to 3, wherein the filament derniers of the filaments differ from fiber bundle to fiber bundle by more than 1.5 dtex.
 5. The filter tow of one of claims 1 to 4, wherein the crimp ratio is greater than 33%, and the number of crimps per cm is greater than
 15. 6. The filter tow of one of claims 1 to 5, wherein the crimp ratio is greater than 35%.
 7. The filter tow of one of the above claims, wherein the crimp ratio is greater than 40%, preferably greater than 42%, and the number of crimps per cm exceeds a value of
 22. 8. The filter tow of crimped, continuous cellulose acetate filaments, wherein the filter tow has a crimp ratio of greater than 40% and the filaments have at least 22 crimps per cm filament length.
 9. The filter tow of claim 8, wherein the crimp ratio is greater than 42% and the filaments have more than 25 crimps per cm.
 10. The filter tow of claim 8 or 9, wherein the filament dernier is greater than 3.3 dtex, and the total dernier is less than 44,000 dtex, preferably less than 40,000 dtex. 